Radial tire with circumferential spirally wound belt layer

ABSTRACT

At least one belt layer in which one or plural rubber-coated belt cords are arranged to be spirally wound almost in parallel with a tire circumferential direction on the outer circumference of a body ply. The belt cord is small in the increase rate of tensile load to stretch rate in the range of a predetermined stretch rate or less, but becomes large in the increase rate of the tensile load in another range exceeding the predetermined stretch rate. Therefore, during a vulcanizing and forming process, the belt cord is expanded together with unvulcanized compounded rubber under a small tensile load, and it does not occur to prevent body ply cords from being expanded, so that a green tire is vulcanized and formed with respective parts thereof being adhered as a result of being expanded to keep a correct shape.

TECHNOLOGICAL FIELD

The present invention relates to a radial tire having belt cords of abelt layer which are arranged to be spirally wound almost in parallelwith a tire circumferential direction.

BACKGROUND ART

For further enhancement of vehicle safety, it has been an increasingstrong demand to improve the braking performance and the steeringstability of tires contacting with a road surface. Another increasingstrong demand is to lighten tires for improved fuel efficiency in termsof environment preservation. To this end, there has been proposed aradial tire described in Japanese unexamined, published patentapplication No. 62-152904. In the known radial tire, body ply cordswhich extend from a tread section to each sidewall section to be turnedup over a bead core of a bead section are arranged in a radial cordfashion (arrangement at an angle in the range of 85 to 90 degreesrelative to the tire circumferential direction), a secondary belt layerin which steel cords are inclined at an angle in the range of 20 to 40degrees relative to the tire circumferential direction is arranged as asingle layer over an outer circumference for a tread section of the bodyply in the tire circumferential direction, and a steel cord or anorganic fiber cord such as Kevlar® which is large in the increase rateof tensile load to stretch rate is spirally wound on an outercircumference side of the oblique belt layer almost in parallel with thetire circumferential direction to form a main belt layer. The beltlayers are to prevent the movement of the body ply by a so-called “hoopeffect” which suppress the radial expansion of the body ply for securingthe steering stability, high-speed endurance and wear resistance whichare characteristic performances of radial tires. As belt cords arrangedin the belt layers, there have been used steel cords and aromaticpolyamide fibers which are large in the increase rate of tensile road tostretch rate for a greater “hoop effect”.

On the other hand, in Japanese unexamined, published patent applicationNo. 8-318706, there is described a radial tire wherein PET (polyethyleneterephthalate) fiber or nylon fiber which is small in the increase rateof tensile load to stretch rate is used for a circumferential belt layerwith the ply structure appropriated.

Further, for a proper “hoop effect” by belt layers, a trial manufacturehas been made of a radial tire, in which as shown in FIGS. 19 and 20,two secondary belt layers 71 and 72 whose respective steel cords areinclined in mutually opposite directions relative to the tirecircumferential direction are arranged on an outer circumference sidefor a tread section of a body ply 70 in the tire circumferentialdirection, and an organic fiber cord is spirally wound on an outercircumference side of the oblique belt layer 72 at the outside almost inparallel with the tire circumferential direction thereby to form a mainbelt layer 73.

In vulcanizing a so-called “green tire” formed with unvulcanizedcompounded rubber in a manufacturing process of tires, the green tire isput in a mold and is vulcanized and formed in the mold with heat andpressure being applied inside. To obtain a target tire shape by thevulcanization and forming, the green tire is expanded within the mold toadd a stretch of several percents or, in dependence upon the type of theforming method, 0.5 to 3%. Where there are employed conventional steelcords or aromatic polyamide fibers which hardly stretch because beinglarge in the increase rate of tensile load to stretch rate, a problemarises in that they cannot be stretched by the pressure applied to thegreen tire up to 0.5 to 3%, whereby the green tire cannot be formed to acorrect shape as a result that the body ply cords are prevented by thenon-stretchable belt cord from being expanded. On the other hand, in thecase of a radial tire which employs as belt cords PET or nylon fibercords which are small in the increase rate of tensile load to stretchrate, there arises a problem that a sufficient hoop effect cannot beobtained after the vulcanization and forming.

The present invention is made to solve the aforementioned problems andis to provide a radial tire with a belt layer in which there iscircumferentially spirally wound a belt cord capable of allowing thegreen tire to expand with a correct shape retained during a vulcanizingand forming process and also capable of exercising a sufficient hoopeffect after the vulcanizing and forming process.

DISCLOSURE OF THE INVENTION

A first invention resides in a radial tire provided with a body plyextending from a tread section to sidewall sections and having at bothside ends thereof turned-up portions which are turned up over bead coresof bead sections; a belt layer wound on an outer circumference of thebody ply in a tire circumferential direction; a tread circumferentiallyarranged on an outer circumference of the belt layer; and bead fillersextending from the bead cores radially outward between the sidewallsections and the turned-up portions of the body ply, wherein the beltlayer comprises at least one spirally wound belt layer in which one orplural belt cords coated with rubber are arranged to be spirally woundalmost in parallel with the tire circumferential direction; and whereinthe belt cord in the spirally wound belt layer is small in the increaserate of tensile load to stretch rate in the range of a predeterminedstretch rate or less but is larger in the increase rate of tensile loadin a range exceeding the predetermined stretch rate.

According to the present invention, since during a vulcanizing andforming process, the belt cord is expanded under a small tensile loadnot to prevent body ply cords from expanding, a green tire is vulcanizedand formed with respective parts thereof being adhered as a result ofbeing expanded to keep a regular shape. Thus, the tire is vulcanized andformed with the belt cord being expanded by the predetermined stretchrate and after the vulcanization, becomes large in the increase rate oftensile road to stretch rate to exercise a sufficient hoop effect.Therefore, the tire can be well maintained in a desired toroidal shape,can be increased in a forward-rearward rigidity in the tirecircumferential direction, can suppress the movement of the treadingsurface, can be enhanced in the braking performance to avoid collisionalaccident, and further can make the shape of a ground contact surface onthe treading surface proper. As a consequence, it becomes possible tomake sudden starting and sharp steering as well as to enhance steeringstability.

A second invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to the aforementioned firstinvention, wherein the belt cord in the spirally wound belt layer is ofthe property that the stretch rate is equal to or greater than 0.5% atthe tensile load of 20 N and that the tensile load at the stretch rateof 3% is equal to or greater than 60 N (preferably, the tensile load atthe stretch rate of 1.5% is equal to or greater than 30 N).

According to the present invention, as is the case of the firstinvention, when the diameter of the green tire is expanded at anexpansion rate in the rage of 0.5 to 3%, the tire is vulcanized andformed with respective parts thereof being adhered as a result of havingthe entire shape thereof maintained properly and after the vulcanizationand forming, becomes large in the hoop effect that the belt layersuppresses the expansion of the body ply.

A third invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to the aforementioned first orsecond invention, wherein the belt cord in the spirally wound belt layeris made of a hybrid cord having a nylon fiber bundle as a core andhaving an aramid fiber bundle twisted therearound.

According to the present invention, it can be realized to provide alight belt cord having a property that the increase rate of tensile loadto stretch rate is small in the range of a predetermined stretch rate orless, but is large in the increase rate of tensile load in a rangeexceeding the predetermined stretch rate.

A fourth invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to the aforementioned first orsecond invention, wherein the belt cord in the spirally wound belt layeris made of a multi-strand steel cord which is made by twisting pluralsteel strands.

According to the present invention, it can be realized to provide a beltcord which is small in the increase rate of tensile load to stretch ratein the range of a predetermined stretch rate or less, but abruptlyincreases the increase rate of tensile load in a range exceeding thepredetermined stretch rate to have a large hoop effect.

A fifth invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to the aforementioned first orsecond invention, wherein the belt cord in the spirally wound belt layeris made of a waved steel cord which is made by giving a wavingprocessing to a steel cord which is made by twisting plural steelfilaments.

According to the present invention, it can be realized to easily adjustthe property of the steel belt cord which is large in the hoop effect asbeing small in the increase rate of tensile load to stretch rate in therange of a predetermined stretch rate or less, but abruptly increasingthe increase rate of tensile load in a range exceeding the predeterminedstretch rate.

A sixth invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to any one of the aforementionedfirst to fifth inventions, wherein the body ply is arranged to be atleast one layer; wherein body ply cords in the body ply are inclined atan angle in the range of 85 to 90 degrees relative to thecircumferential direction; and wherein the belt layer arranged on theouter circumference of the body ply comprises an oblique belt layer inwhich a belt cord made of steel is arranged as one layer to be inclinedat an angle in the range of 10 to 40 degrees relative to thecircumferential direction and at least one spirally wound belt layer inwhich a belt cord is spirally arranged on an outer circumference of theoblique belt layer almost in parallel with the tire circumferentialdirection.

According to the present invention, in addition to the effects of thefirst invention, it can be realized to lighten the radial tire, toenhance the braking performance and the steering stability of the tireand to maintain the high-speed endurance and the wear resistance.

A seventh invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to any one of the aforementionedfirst to third inventions, wherein the body ply is arranged to be atleast one layer; wherein body ply cords in the body ply are inclined atan angle in the range of 85 to 90 degrees relative to thecircumferential direction; and wherein the belt layer arranged on theouter circumference of the body ply comprises two oblique belt layers inwhich belt cords made of steel are arranged to be inclined in the samedirection at the same angle in the range of 30 to 60 degrees(preferably, in the range of 40 to 50 degrees) relative to thecircumferential direction and at least one spirally wound belt layer inwhich a rubber-coated belt cord made of a hybrid cord is spirallyarranged on outer circumferences of the oblique belt layers almost inparallel with the tire circumferential direction; and wherein of the twooblique belt layers, one oblique belt layer is in the range of 40 to 70%of the other oblique belt layer in width and is circumferentiallyarranged at the center portion in the direction of the radial tirewidth.

According to the present invention, by the hoop effects of the two beltlayers inclined in the same direction and the circumferentially spirallywound belt layer, it can be realized to enhance the braking performance,the steering stability and the wear resistance of the tire and inparticular, to enhance the braking effect on wet roads. It is usual thatin the steel belt arranging structure, two belt layers are arranged tohave respective belt cords wound and inclined in opposite directions andcrossed mutually. In that case, respective belt cords interfere to betwisted, and this causes the distribution of the ground contact pressureon the ground contact surface to be uneven and not to become flat, sothat though the ground contact area appears unchanged, there isdecreased a substantial ground contact area which works for the brakingperformance and the steering stability. On the other hand, in thepresent invention, since the two oblique belt layers are wound with thebelt cords being inclined at the same angle in the same direction, thedistribution of the ground contact pressure on the ground contactsurface becomes even and flat, so that though the ground contact areaappears unchanged, the substantial ground contact area which works forthe braking performance and the steering stability can be made to belarge thereby to improve the braking performance and the steeringstability. Further, since one oblique belt layer of the two oblique beltlayers is in the range of 40 to 70% of the other oblique belt layer inwidth and is circumferentially arranged at the center portion in thedirection of the radial tire width, the floating motion which is liableto occur at the center portion of the tread surface when a suddenincrease in load is brought about upon braking can be suppressed, andthe flexibility of the tread section can be maintained.

An eighth invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to the aforementioned seventhinvention, wherein an organic fiber cord made of a hybrid cord is coatedwith rubber and is arranged to be spirally wound between the obliquebelt layers and the outer circumference of the body ply almost inparallel with the circumferential direction thereby to form a ply underthe oblique belt layers; and wherein the arrangement of the organiccords in the ply under the oblique belt layers is made to be densifiedat shoulder portions on the side edges in the direction of the radialtire width and is made to be loose at the center portion.

According to the present invention, the ply under the oblique beltlayers is arranged on the outer circumference of the body ply to extendfrom the shoulder portions to the center portion, and thus, when asudden load acts upon braking, the ply under the oblique belt layerssuppresses the separations between the body ply cords, so thatdifferences in tension among the body ply cords can be lessened toenhance the braking performance. Further, the cord in the ply under theoblique belt layers is arranged to be densified at the shoulder portionsand to be loose at the center portion, and thus, when an inside pressureis applied to the tire, the ground contact state of the shoulderportions can be kept stable because of the densified arrangement of theorganic fiber cord thereat, so that it can be realized to remarkablyenhance the steering stability and the braking performance of the tireon wet roads and also to enhance the braking performance and the wearresistance on dry roads. At the center portion, the organic fiber cordis arranged to be loose, so that the separations between the body plycords can be suppressed without deteriorating the flexibility.

A ninth invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to the aforementioned seventhinvention, wherein a ply under the oblique belt layers is composed of awaved steel cord coated with rubber and arranged to be spirally woundand to be densified between the oblique belt layers and the outercircumference of the body ply at shoulder portions on the side edges inthe direction of the radial tire width almost in parallel with thecircumferential direction and an organic fiber cord made of a hybridcord coated with rubber and arranged to be spirally wound and to beloose between the oblique belt layers and the outer circumference of thebody ply at the center portion almost in parallel with thecircumferential direction.

According to the present invention, when a sudden load acts uponbraking, the ply under the oblique belt layers suppresses theseparations between the body ply cords, so that differences in tensionamong the body ply cords can be lessened to enhance the brakingperformance. Further, since the waved steel cord is employed as thematerial at the shoulder portions of the ply under the oblique beltlayers, the shoulder portions of the radial tire are increased inrigidity and further are stabilized in the ground contact state uponsudden braking. Thus, remarkable improvements can be made in the brakingperformance and the steering stability of the tire on wet roads, andimprovements can also be made in the braking performance and the wearresistance on dry roads. By the organic fiber cord spirally wound to beloose, the floating of the center portion can be suppressed to theextent that the flexibility cannot be deteriorated.

A tenth invention resides in a radial tire with the circumferentiallywound belt layer according to the aforementioned seventh invention,wherein a cord which is small in the increase rate of tensile load tostretch rate in the range of a predetermined stretch rate or less but islarge in the increase rate of tensile load in a range exceeding thepredetermined stretch rate is coated with rubber and is arranged to bespirally wound between the oblique belt layers and the outercircumference of the body ply almost in parallel with thecircumferential direction thereby to form a ply under the oblique beltlayers, and wherein the cord in the ply under the oblique belt layers isarranged to be densified at the shoulder portions on the side edges inthe direction of the radial tire width and at the center portion, but tobe loose at a portion between each shoulder portion and the centerportion.

According to the present invention, the ply under the oblique beltlayers is arranged on the outer circumference of the body ply to extendfrom the shoulder portions to the center portion, and thus, when asudden load acts upon braking, the ply under the oblique belt layerssuppresses the separations between the body ply cords, so thatdifferences in tension among the body ply cords can be lessened toenhance the braking performance. In addition, since the cord in the plyunder the oblique belt layers is arranged to be densified at theshoulder portions and the center portion and to be loose at the portionbetween each shoulder portion and the center portion, the loose portionserves for lightening the tire, and at the same time, the ground contactstate is kept to be always stable at the shoulder portions and thecenter portion where the cord arrangement is densified. Thus, remarkableimprovements can be made in the steering stability and the brakingperformance of the tire on wet roads, and improvements can also be madein the braking performance and the wear resistance on dry roads. Becauseof being densified at the center portion, the floating of the centerportion in high speed traveling can be suppressed to improve thesteering stability in high speed traveling.

An eleventh invention resides in a radial tire with thecircumferentially spirally wound belt layer according to theaforementioned tenth invention, wherein a cord which is small in theincrease rate of tensile load to stretch rate in the range of apredetermined stretch rate or less but is large in the increase rate oftensile load in a range exceeding the predetermined stretch rate iscoated with rubber and is arranged to be spirally wound between thespirally wound belt layer and the oblique belt layers almost in parallelwith the circumferential direction thereby to form a ply under thespirally wound belt layer, and wherein the cord in the ply under thespirally wound belt layer is arranged to be densified at the shoulderportions on the side edges in the direction of the radial tire width andat the center portion, but to be loose at the portion between eachshoulder portion and the center portion.

According to the present invention, when a sudden load acts uponbraking, the ply under the oblique belt layers suppresses theseparations between the body ply cords, so that differences in tensionamong the body ply cords can be lessened to enhance the brakingperformance. In addition, the shoulder portions and the center portionare heightened in rigidity, and the ground contact state at the shoulderportions upon sudden braking is stabilized. Thus, remarkableimprovements can be made in the braking performance and the steeringstability of the tire on wet roads, and improvements can also be made inthe braking performance and the wear resistance on dry roads. Thefloating of the center portion in high speed traveling can be suppressedto improve the steering stability in high speed traveling. Furthermore,since the ply under the spirally wound belt layer is arranged betweenthe circumferentially spirally wound belt layer and the oblique beltlayers, the hoop effect on the body ply can further be strengthenedthereby to improve the steering stability and the wear resistance. Inaddition, since the steel cords in the oblique belt layers aresandwiched and tied by the ply under the spirally wound belt layer andthe ply under the oblique belt layers to be densified at the shoulderportions and the center portion, the movement of the steel cords in theoblique belt layers is restricted during the rolling of the tire,whereby reduction can be made in the noise in the car on rough roads.

A twelfth invention resides in a radial tire with the circumferentiallyspirally wound belt layer according to any one of the aforementionedsixth to eleventh inventions, wherein both side portions of the spirallywound belt layer cover both side portions of the oblique belt layers.

According to the present invention, it can be prevented that both sideportions of the cords in the oblique belt layers are brought intocontact with the tread internal surface thereby to become a core fortroubles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a radial tire, taken in a tire radialdirection, having circumferentially spirally wound belt layers in afirst embodiment;

FIG. 2 is a figure showing a body ply and the belt layers in the firstembodiment;

FIG. 3 is a figure showing the states of a belt cord and belt cordscoated with unvulcanized compounded rubber;

FIG. 4 is a graph showing tensile loads of belt cords relative tostretch rates;

FIG. 5 is a perspective view of a multi-strand steel cord;

FIG. 6 is a sectional view of a radial tire, taken in a tire radialdirection, having circumferentially spirally wound belt layers in asecond embodiment;

FIG. 7 is a figure showing body plies and the belt layers in the secondembodiment;

FIG. 8 is a perspective view of a hybrid cord;

FIG. 9 is a sectional view of a radial tire, taken in a tire radialdirection, having circumferentially spirally wound belt layers in athird embodiment;

FIG. 10 is a figure showing a body ply and the belt layers in the thirdembodiment;

FIG. 11 is a sectional view of a radial tire, taken in a tire radialdirection, having a circumferentially spirally wound belt layer in afourth embodiment;

FIG. 12 is a figure showing the arrangement of a body ply, the obliquebelt layers and a ply under the oblique belt layers in the fourthembodiment;

FIG. 13 is a sectional view of a radial tire, taken in a tire radialdirection, having a circumferentially spirally wound belt layer in afifth embodiment;

FIG. 14 is a figure showing the arrangement of a body ply, the obliquebelt layers and a ply under the oblique belt layers in the fifthembodiment;

FIG. 15 is a perspective view of a steel cord given a waving processing;

FIG. 16 is a figure showing the arrangement of a body ply, oblique beltlayers and a ply under the oblique belt layers in a sixth embodiment;

FIG. 17 is a figure showing the arrangement of a body ply, oblique beltlayers, a ply under the oblique belt layers and a ply under a spirallywound belt layer in a seventh embodiment;

FIG. 18 is a figure showing an eighth embodiment wherein the belt layersin FIG. 17 are modified to include one oblique belt layer;

FIG. 19 is a sectional view of a prior art radial tire taken in a tireradial direction; and

FIG. 20 is a figure showing the arrangement of a body ply and beltlayers of the prior art radial tire.

PREFERRED EMBODIMENTS TO PRACTICE THE INVENTION

Hereafter, description will be made with reference to the accompanyingdrawings as to a radial tire having circumferentially spirally woundbelt layers in the first embodiment in which the present invention isapplied to a radial tire for passenger cars. As shown in FIGS. 1 and 2,the radial tire 1 is composed of a tread section 2, sidewall sections 3at both sides of the tire radially bent from both sides of the treadsection 2 toward the tire axis, and annular bead sections 4 constitutinginner circumferences of the respective sidewall sections 3. The beadsections 4 at the both sides are provided with bead cores 5 eachconstituted by annually winding a steel cord. A body ply 6 extends fromthe tread section 2 to pass through the sidewall sections 3 at the bothsides and then to be turned up over the respective bead cores 5, so thatthe body play 6 is suspended between the both side beads. In the bodyply 6, body ply cords 7 are coated with rubber and are arranged at anoblique angel in the range of 85 to 90 degrees relative to the tirecircumferential direction. With the arrangement of the body ply cords atthe oblique angle in the range of 85 to 90 degrees as well as with anexcellent hoop effect of a spirally wound belt layer 10 having the beltcord 9 spirally wound in the tire circumferential direction, the radialtire can easily be kept in a desired toroidal shape and thus, can beexcellent in the braking performance and the steering stability.

The spirally wound belt layer 10 comprising two layers 9 a and 9 b isformed on the outer circumference of the body ply 6, and in each of thelayers, a belt cord 9 having an appropriate tensile rigidity is coatedwith rubber and is spirally wound at an oblique angle in the range of 0to 9 degrees almost in parallel with the tire circumferential direction.In this case, the spirally wound belt layer 10 may be formed by spirallywinding a single belt cord 9 in turn for the first layer 9 a and thesecond layer 9 b with the circumferential surface of the belt cord 9coated with unvulcanized compounded rubber as shown in FIG. 3( a). Or,the spirally wound belt layer 10 may be constituted by spirally windingplural belt cords simultaneously in turn for the first layer 9 a and thesecond layer 9 b with the plural belt cords coated with unvulcanizedcompounded rubber in a parallel arrangement to form a ribbon as shown inFIG. 3( b). Or, the spirally wound belt layer 10 may be constituted byspirally winding, by one turn, plural belt cords of double layers whichare coated with unvulcanized compounded rubber in parallel arrangementswith one layer for the first layer 9 a piled up on the other layer forthe second layer 9 b as shown in FIG. 3( c). A manufacturing method ofspirally winding the rubber-coated belt cords 9 on the outercircumference of the body ply 6 can be implemented by a known methoddescribed in, e.g., Japanese unexamined, published patent applicationNo. 2002-137607 and is therefore omitted from being described.

As referred to later, finally, the belt cords 9 are spirally wound toform two layers almost in parallel with the tire circumferentialdirection with the increase rate of tensile load to stretch rate kept tobe large. Thus, the hoop effect that the spirally wound belt layer 10prevents the body ply 6 from expanding becomes large to keep the tireproperly in a desired toroidal shape, and the forward-rearward rigidityin the tire circumferential direction can be increased to suppress thedisplacement of the tread surface, so that the braking performance canbe enhanced to prevent collisional accidents. Further, the steeringstability can be improved since sudden starting and sharp steeringbecome possible.

An inner liner 11 is arranged on the internal surface of the body ply 6,a tread 12 is wound on the outer surface of the spirally wound beltlayers 10, a bead filler 13 is arranged between each sidewall sectionand each turned-up portion of the body ply 6 to extend from each beadcore 5 radially outward of the tire, and a side tread 20 is arranged ateach of the sidewall section 3.

A green tire whose structure roughly described above is formed withunvulcanized compounded rubber is put into a vulcanizing mold, and then,an expansible shape fit in the internal surface of the inner liner 11 isexpanded while they are heated up to about 200 degrees centigrade,whereby the inner liner 11, the body ply 6, the spirally wound beltlayers 10 and the tread 12 are adhered and formed together through thevulcanization of the unvulcanized compounded rubber. The expansion ofthe expansible shape causes the tire outer diameter to expand at anexpansion rate in the range of 0.5 to 3%, and therefore, unless the beltcords 9 can be expanded under a small tensile load during this expansionprocess, the correct tire shape cannot be kept as a result that the bodyply cords 7 are prevented by the unexpanded belt cords 9 from beingexpanded.

In view of the above, various tests were carried out with tirevulcanization and forming conditions changed. From the test results, itwas found that in the case of using for the belt cords 9 a cord whichwas manufactured to have the property that as shown in FIG. 4, thestretch rate at the tensile load of 20 N (newton) is equal to or greaterthan 0.5% and that the tensile load at the stretch rate of 3% is equalto or greater than 60 N (preferably, the tensile load at the stretchrate of 1.5% is equal to or greater than 30 N), the inner liner 11, thebody ply 6, the spirally wound belt layer 10 and the tread 12 werereliably adhered and vulcanized with the entire shape of the tire keptproperly during the expansion of the expansible shape. In this case,since the tire is vulcanized and formed with the belt cord 9 beingstretched at a stretch rate in the range of 0.5 to 3%, the vulcanizedand formed tire is heightened in the tensile rigidity of the belt cord 9and is increased in the hoop effect that the spirally wound belt layer10 prevents the body ply 6 from expanding. Thus, the tire can be keptproperly in a desired toroidal shape, so that improvements can be madein the braking performance and the steering stability.

The belt cord 9 used as one example in the first embodiment was a steelcord with a structure of 3×3×0.175. This is a multi-strand steel cord 19having a cord diameter of 0.81 Φmm (millimeter) which cord is structuredby twisting three steel filaments 18 a of the diameter 0.175 Φmm to makea strand 18 b and then by twisting three strands 18 b so made at a shortpitch. As indicated at (a) in FIG. 4, the property of the tensile loadrelative to the stretch rate of the multi-strand steel cord 19 indicatesthe stretch rate of 0.6% larger than 0.5% at the tensile load of 20 N,attains the tensile load of 350 N at the stretch rate of 1.5%. At astretch rate equal to or less than about 0.5% which is a predeterminedvalue, the increase rate of tensile load to stretch rate becomes small,but becomes large as the stretch rate exceeds the predetermined value.

The belt cord 9 used in the spirally wound belt layer 10 may be a wavedsteel cord 50 shown in FIG. 15. This waved steel cord 50 is made bywaving a steel cord with the structure of 1×3×0.27 (i.e., a cord of 0.58Φmm cord diameter which is made by twisting three steel filaments 53each having 0.27 Φmm). In the waved steel cord 50, by altering the waveshape, it can be realized relatively easily to adjust a desired propertythat the increase rate of tensile load to stretch rate is small in asmall load range but is large in a stretch rate range exceeding thepredetermined value.

By the selection of the twisting method, any aramid cord which is largein the increase rate of tensile load to stretch rate and small in thestretch rate of each fiber itself or any steel cord can be made to alight belt cord having a desired property that the increase rate oftensile load to stretch rate is small in a small load range but is largein a stretch rate range exceeding the predetermined value.

The tensile tests for cords were performed by using a Model AGS-500Amachine made by Shimadzu Corporation, Kyoto, Japan within a room whichis maintained at 25 (room temperature)±3 degrees centigrade and 65% inrelative humidity under the conditions of the grip-to-grip interval of250 mm and the stretching speeds of 300 mm/min for organic fiber cordsand 125 mm/min for steel cords. As cord samples, there were used thosewhich were kept at a constant temperature (20±2 degrees centigrade)within a tightly-closed bottle filled with 36% sulfuric acid.

In a second embodiment, as shown in FIGS. 6 and 7, body ply cords 22 arecoated with rubber and are parallel arranged in a body ply 21 composedof two layers 22 a and 22 b at an oblique angle in the range of 85 to 90degrees relative to the tire circumferential direction. A two-layer beltlayer 23 is arranged on the outer circumference of the body ply 21. Thefirst layer inside in the tire radial direction is an oblique belt layer25, in which belt cords 24 made of steel are coated with rubber and arearranged in a single layer to be inclined at an angle in the range of 10to 40 degrees, preferably at an angle of 20 degrees. The second layer isa spirally wound belt layer 26 in which one or more belt cords 9 coatedwith rubber 8 as shown in FIG. 3( a) or 3(b) are spirally wound in asingle layer at an oblique angle in the range of 0 to 9 degrees almostparallel with the tire circumferential direction.

For the belt cord 9 in the spirally wound belt layer 26, there is used ahybrid cord 29 (refer to FIG. 8) which is structured by twisting anaramid fiber bundle 27 (e.g., Aramid 1100 dtex) typifying aromaticpolyamid fiber cords with a nylon fiber bundle 28 (e.g., Nylon 940 dtex)of aliphatic polyamide system. As indicated at (b) in FIG. 4, theproperty of tensile load to stretch rate of the hybrid cord 29 is suchthat the tensile load attains 80 N larger than 60 N at the stretch rateof 3% and attains 300 N at the stretch rate of 6% and that the increaserate of tensile load to stretch rate becomes small in a stretch raterange of about 3% or less which is a predetermined value, but becomeslarge in another stretch rate range exceeding the predetermined value.The tensile load to stretch rate can be increased by bundling pluralhybrid cords 29. In the later mentioned second embodiment, three hybridcords 29 were bundled to make a cord of 1.08 Φmm in cord diameter foruse as the belt cord 9. It is possible to vary the aforementionedpredetermined value by adjusting the twisting structure.

In the second embodiment, the belt layer 23 comprise an oblique beltlayer 25 arranging therein high rigidity steel cords 24 to incline at asmall angle (in the range of 10 to 40 degrees) relative to thecircumferential direction and a spirally wound belt layer 26 spirallyarranging one or more rubber-coated belt cords 9 almost in parallel (inthe range of 0 to 9 degrees) with the tire circumferential direction.Thus, the hoop effect can be exercised very strongly on the body ply 6,so that improvements can be made in the braking performance and thesteering stability. Further, since the oblique belt layer 25 includingthe steel cords 24 is one layer, it can be realized to lighten the tireas well as to achieve an advantage of a small rolling resistance. Inaddition, since the belt cord 9 used for the spirally wound belt layer26 is made of a bundle of plural hybrid cords 29 each making a core withthe nylon fiber bundle 28 and having the aramid fiber bundle 27 twistedtherearound, the belt cord 9 can be obtained as that which is proper inthe increase rate of tensile load to stretch rate and which is light.Therefore, it can be also realized to lighten the tire as well as toreduce the occurrence of defectives in the tire production.

In a third embodiment, as shown in FIGS. 9 and 10, body ply cords 32 areparallel arranged in a single body ply 31 to be coated in a rubber layerat an oblique angle in the range of 85 to 90 degrees relative to thetire circumferential direction. Two oblique belt layers 33 and 34 arearranged on the outer circumference of the body plies 31 in thecircumferential direction. In the two oblique belt layers 33 and 34,respective belt cords 35 made of steel are arranged at an angle in therange 30 to 60 degrees (preferably, in the range of 40 to 50 degrees)relative to the circumferential direction with themselves extending atthe same oblique angle in the same direction. Of the two oblique beltlayers 33 and 34, the oblique belt layer 34 at the outer side is made tobe in the range of 40 to 70% of the oblique belt layer 33 at the innerside in width and is arranged at the center portion in the direction ofthe radial tire width. Conversely, the width of the oblique belt layer33 at the inner side may be made to be in the range of 40 to 70% of thatof the oblique belt layer at the outer side. On the outer circumferenceof the two oblique belt layers 33 and 34, a spirally wound belt layer 36is formed by spirally winding a belt cord 9 almost in parallel with thetire circumferential direction. The belt cord 9 is made by bundling andrubber-coating three hybrid cords 29 each of which is small in theincrease rate of tensile load to stretch rate in a smaller stretch raterange than a predetermined stretch rate range but is large in theincrease rate of tensile load in another range exceeding thepredetermined stretch rate. A belt layer 37 is composed of the twooblique belt layers 33, 34 and the spirally wound belt layer 36.

In the third embodiment, the two oblique belt layers 33, 34 in which thebelt cords 35 made of steel are arranged to incline in the samedirection at the same oblique angle relative to the circumferentialdirection are wound at the outer circumference of the body ply 31, andthe belt cord 9 made of the hybrid cord 29 and coated with rubber isarranged at the outer circumference of the oblique belt layers 33, 34 ina spiral winding fashion to extend almost in parallel with the tirecircumferential direction. Thus, the ground contact surface is made tobe even and flat, whereby also with the hoop effect by thecircumferentially spirally wound belt layer, improvements can be made inthe steering stability, the wear resistance and the braking performanceof the tire and particularly, in the braking performance on wet roads.

It has been a practice that in the structural arrangement of steelbelts, two belt layers are arranged to be wound with respective beltcords oriented in opposite directions to cross mutually. In that case,respective belt cords interfere to be twisted, and this causes thedistribution of the ground contact pressure on the ground contactsurface to be uneven and not to become flat, so that though the groundcontact area appears unchanged, there is decreased a substantial groundcontact area which works for the braking performance and the steeringstability. On the other hand, in the third embodiment, since the twooblique belt layers 33, 34 are wound with the respective belt cordsinclined at the same angle in the same direction, the distribution ofthe ground contact pressure on the ground contact surface becomes evenand flat, so that though the ground contact area appears unchanged, thesubstantial ground contact area which works for the braking performanceand the steering stability can be made to be large thereby to improvethe braking performance and the steering stability.

Further, since one oblique belt layer 34 of the two oblique belt layers33, 34 is in the range of 40 to 70% of the other oblique belt layer 33in width and is circumferentially arranged at the center portion in thedirection of the radial tire width, the floating motion which is liableto occur at the center portion of the tread surface when a suddenincrease in load is brought about by braking can be suppressed, and theflexibility of the tread surface can be maintained. Where the width ofthe oblique belt layer 34 is made to be narrower than the 40% width ofthe oblique belt layer 33, the center portion of the tread sectionbecomes liable to float, and where it is made to be more than the 70%width of the oblique belt layer 33, the flexibility of the tread sectionis deteriorated to make the riding comfortableness and the noise in thecar worse.

Fourth and fifth embodiments which will be described hereafter aredifferent from the third embodiment only in the structure that a ply 41under the oblique belt layers is arranged between the oblique belt layer33 and the outer circumference of the body ply 31. Therefore, thisdifference will be described, and other components given the samereference numerals as those in the third embodiment will be omitted frombeing described.

In the fourth embodiment, as shown in FIGS. 11 and 12, an organic fibercord 41 made of a single hybrid cord 29 or plural twisted hybrid cords29 is coated with rubber and is arranged to be spirally wound betweenthe oblique belt layers 33, 34 and the outer circumference of the bodyply 31 almost in parallel with the circumferential direction thereby toform a ply 42 under the oblique belt layers. The number of the organicfiber cords 41 arranged in the ply 42 under the oblique belt layers issuch that each of the shoulder portions 43 on the side edges in thedirection of the radial tire width is densified to have the organicfiber cords 41 in the number of 14±10 or so per 10 mm and that thecenter portion 44 is made to be loose and to have the organic fibercords 41 of the number in the range of 40 to 85%, desirably in the rangeof 50 to 80% of those at each shoulder portion 43. At the center portion44, the organic fiber cords 41 may be arranged to be gradually loosetoward the tire center. When an expansible shape is expanded with thegreen tire placed in a vulcanizing mold, the organic fiber cord 41 madeof the hybrid cord 29 moderately stretches not to deteriorate the entireshape of the tire.

In this manner, the ply 42 under the oblique belt layers is arranged toextend from the shoulder portions 43 to the center portion 44 on theouter circumference of the body ply 31. Thus, when a sudden load actsupon braking, the ply 42 under the oblique belt layers suppresses theseparations between the body ply cords 32, so that differences intension among the body ply cords 32 can be lessened to enhance thebraking performance. Further, when having an inside pressure appliedthereto, the tire can be further stabled to keep its predeterminedshape. In addition, the organic fiber cord 41 in the ply 42 under theoblique belt layers is arranged to be densified at the shoulder portions43 and to be loose at the center portion 44. Thus, when the insidepressure is applied to the tire, the ground contact state of theshoulder portions 42 can be kept stable because of the densifiedarrangement of the organic fiber cord 41 thereat. As a consequence, itcan be realized to remarkably enhance the steering stability and thebraking performance of the tire on wet roads and also to enhance thebraking performance and the wear resistance on dry roads. While thefloating of the center portion 44 due to the centrifugal force issuppressed primarily by the belt layer 37, the organic fiber cord 41 isarranged to be loose at the center portion 44, whereby the separationsbetween the body ply cords 32 and the floating due to the centrifugalforce can be suppressed without deteriorating the flexibility.

In the fifth embodiment shown in FIGS. 13 and 14, a waved steel cord 50given a waving processing as shown in FIG. 15 is coated with rubber andis arranged to be spirally wound between the oblique belt layers 33, 34and the outer circumference of the body ply 31 at each of the shoulderportions 43 almost in parallel with the circumferential direction. Asthe waved steel cord 50, it is preferred to use one which meets therequirements that a value H/d which is obtained by dividing theamplitude H (i.e., the distance between the top of a wave and the top ofanother wave directed in the opposite direction) by a cord diameter dcomes in the range of 1.1 to 3.0 and that a value L/d which is obtainedby dividing the wavelength L by the cord diameter comes in the range of2 to 100. In the later described fifth embodiment, a cord having a1×3×0.20 structure (a cord which was made by twisting three filaments 53each with 0.20 Φmm) and also having its cord diameter of 0.424 Φmmbefore the waving processing was given the waving processing to beformed to a wave shape with the amplitude H=0.68 mm and the wavelengthL=2.0 mm, and the cord so waved was used.

When an expansible shape is expanded with the green tire placed in avulcanizing mold, the waved steel cords 50 are moderately stretched notto deteriorate the entire form of the tire. At the center portion 44, anorganic fiber cord 51 made of the single hybrid cord 29 or the pluraltwisted hybrid cords 29 is spirally wound and arranged to be loosebetween the oblique belt layers 33, 34 and the outer circumference ofthe body ply 31 almost in parallel with the circumferential direction. Aply 52 under the oblique belt layers is formed with the steel cord 50arranged at the shoulder portions 43 and with the organic fiber cord 51arranged at the center portion 44.

According to the fifth embodiment, as is the case of the fourthembodiment, when a sudden load acts upon braking, the ply 52 under theoblique belt layers suppresses the separations between the body plycords 32, so that differences in tension among the body ply cords 32 canbe lessened to enhance the braking performance. Further, since the wavedsteel cord 50 is employed for the material at the shoulder portions 43of the ply 52 under the oblique belt layers, the shoulder portions 43are increased in rigidity and are further stabilized in the groundcontact state upon sudden braking. Thus, remarkable improvements can bemade in the braking performance and the steering stability of the tireon wet roads, and improvements can also be made in the brakingperformance and the wear resistance on dry roads. By the organic fibercord 51 spirally wound to be loose, the floating of center portion 44can be suppressed to the extent that the flexibility cannot bedeteriorated.

Although in the fourth and fifth embodiments, the organic fiber cords41, 51 each made of the single hybrid cord 29 or the plural twistedhybrid cords 29 are used to form the plies 42, 52 under the oblique beltlayers respectively, an ordinary organic fiber cord, that is, a cord(e.g., Nylon 1400detex/2) made by twisting two nylon fiber bundles maybe used to form the plies 42, 52 under the oblique belt layers.

In a sixth embodiment, as shown in FIG. 16, the waved steel cord 50(refer to FIG. 15) given the waving processing is coated with rubber andis arranged to be spirally wound between the oblique belt layer 33 andthe outer circumference of the body ply 31 almost in parallel with thecircumferential direction thereby to form a ply 62 under the obliquebelt layers. The number of the steel cords 50 arranged in the ply 62under the oblique belt layers is such that each of the shoulder portions43 on the side edges in the direction of the radial tire width and thecenter portion 64 is densified to have the steel cords 50 in the numberof 14±10 or so per 10 mm and that a portion between each shoulderportion 43 and the center portion 64 is made to be loose and to have thesteel cords 50 of the number in the range of 40 to 85%, preferably inthe range of 50 to 80% of those at each shoulder portion 43. When anexpansible shape is expanded with the green tire placed in a vulcanizingmold, the waved steel cord 50 is moderately stretched not to deterioratethe entire shape of the tire. In place of the waved steel cords 50, theorganic fiber cord 51 made of the hybrid cord 29 may be used for thecord in the ply 62 under the oblique belt layers.

According to the sixth embodiment, the ply 62 under the oblique beltlayers is arranged on the outer circumference of the body ply 31 toextend from the shoulder portions 43 to the center portion 64, and thus,when a sudden load acts upon braking, the ply 62 under the oblique beltlayers suppresses the separations between the body ply cords 32, so thatdifferences in tension among the body ply cords 32 can be lessened toenhance the braking performance. In addition, since the cord in the ply62 under the oblique belt layers is arranged to be densified at theshoulder portions 43 and the center portion 64 but to be loose at theportion between each shoulder portion 43 and the center portion 64, theloose areas help to lighten the tire, and at the same time, the groundcontact state is kept to be always stable at the shoulder portions 43and the center portion 64 where the cord arrangement is densified. As aconsequence, remarkable improvements can be made in the steeringstability and the braking performance of the tire on wet roads, andimprovements can also be made in the braking performance and the wearresistance on dry roads. Because the cord arrangement is densified atthe center portion 64, the floating of the center portion in high speedtraveling can be suppressed to improve the steering stability in highspeed traveling. Accordingly, the tire is suitable for use as those forsports-oriented vehicles in which importance is placed on the steeringstability.

A seventh embodiment shown in FIG. 17 is such that in the sixthembodiment, the waved steel cord 50 (refer to FIG. 15) given the wavingprocessing is coated with rubber and is also arranged to be spirallywound between the circumferentially spirally wound belt layer 36 and theoblique belt layers 33, 34 almost in parallel with the circumferentialdirection thereby to form a ply 65 under the spirally wound belt layer.The number of the steel cords 50 arranged in the ply 65 under thespirally wound belt layer is such that each of the shoulder portions 43on the side edges in the direction of the radial tire width and thecenter portion 64 is densified to have the steel cords 50 in the numberof 14±10 or so per 10 mm and that the portion between each shoulderportion 43 and the center portion 64 is made to be loose and to have thesteel cords 50 of the number in the range of 40 to 85%, preferably inthe range of 50 to 80% of those at each shoulder portion 43. When anexpansible shape is expanded with the green tire placed in a vulcanizingmold, the waved steel cord 50 is moderately stretched not to deterioratethe entire shape of the tire. In place of the waved steel cord 50, theorganic fiber cord 51 made of the hybrid cord 29 may be used for thecord in the ply 65 under the spirally wound belt layer.

According to the seventh embodiment, when a sudden load acts uponbraking, the ply 62 under the oblique belt layers suppresses theseparations between the body ply cords 32, so that differences intension among the body ply cords 32 can be lessened to enhance thebraking performance. In addition, the shoulder portions 43 and thecenter portion 64 are heightened in rigidity, and the ground contactstate at the shoulder portions 43 upon sudden braking is stabilized. Asa consequence, remarkable improvements can be made in the brakingperformance and the steering stability of the tire on wet roads, andimprovements can also be made in the braking performance and the wearresistance on dry roads. The floating of the center portion in highspeed traveling can be suppressed to improve the steering stability inhigh speed traveling. Furthermore, since the ply 65 under the spirallywound belt layer is arranged between the circumferentially spirallywound belt layer 36 and the oblique belt layers 33, 34, the hoop effecton the body ply 32 can further be strengthened thereby to improve thesteering stability and the wear resistance. In addition, since the steelcords 35 of the oblique belt layers 33, 34 are tightly sandwiched andtied by the ply 65 under the spirally wound belt layer and the ply 62under the oblique belt layers at the shoulder portions 43 and the centerportion 64, the movements of the steel cords 35 in the oblique beltlayers 33, 34 are restricted during the rolling of the tire, whereby thenoise in the car can be reduced.

An eighth embodiment shown in FIG. 18 is such that in the seventhembodiment, the oblique belt 34 is removed to leave the oblique belt 33only. In comparison with the seventh embodiment, the eighth embodimentis advantageous in lightening the tire though the operation and effectof the oblique belt 34 can no longer be expected.

Next, the tires of Examples 1 to 5 were manufactured based on theforegoing first to fifth embodiments and were compared with a trial tirewhich is shown as prior art in FIGS. 19 and 20 to show the test resultsof comparison in performance. Used for compassion were low-aspectpneumatic tires each having a tire size of 215/45ZR17, and tests werecarried out for dry-road braking performance, wet-road brakingperformance, steering stability and wear resistance. Each test resultwas evaluated using an index compared with the index 100 given to theresult of the prior art trial tire. Values of the test results areindicated in Table 1, wherein a larger index means a better performance.

For dry-road braking test, the compared tires were attached to a carwith an anti-skid brake system (ABS) and were tested by making the carrun on a test course. The car running on a dry straight road at thespeed of 100 km/h was suddenly braked to the extent that the ABS works,and the braking distance through which the car ran until stopped wasmeasured.

For wet-road braking test, the compared tires were attached to the carwith the ABS. The car running on a wet road of about 1 mm water depth atthe speed of 100 km/h was suddenly braked to the extent that the ABSworks, and the braking distance through which the car ran until stoppedwas measured.

For steering stability, the compared tires were attached to rims of thetype 7JJ, and air was filled in each tire to make the inside pressure240 kPa. The rims were attached to a car with an engine having thedisplacement of 2000 cc (i.e., 2-liter engine), and the car was steeredto go straight ahead and to make lane changes at speeds in the range of60 to 180 km/h on a circuit course. The steering stability was evaluatedin dependence on the driver's feeling.

For wear resistance, after having had the compared tires attachedthereto, the car was made to run on highways and ordinary roads at an1-to-1 ratio through the distance of 15,000 km, and thereafter, the wearstates of the tires were evaluated by measuring the remaining treadgroove depth of each tire.

TABLE 1 Exam- Exam- Exam- Exam- Exam- Prior Art ple ple ple ple pleTrial Tire 1 2 3 4 5 Dry Braking 100 101 102 102 104 104 PerformanceWetBraking 100 103 104 107 111 115 Performance Steering 100 101 102 105107 107 Stability Wear 100 100 100 102 102 102 Resistance

The foregoing first to fifth embodiments respectively employ the beltcords 9 whose properties of tensile load to stretch rate arerespectively the properties (a) and (b) shown in FIG. 4. In amodification, a belt cord may be selected from belt cords which have aproperty between the properties (a) and (b), that is, a property thatthe stretch rate indicates 0.5% or more at the tensile load of 20 N andthat the tensile load indicates 60N or more at the stretch rate of 3%(preferably, the tensile load indicates 30N or more at the stretch rateof 1.5%) and which meet the vulcanizing and forming conditions. In thiscase, when expanded in its outer diameter at an expansion rate in therange of 0.5 to 3%, the green tire can be vulcanized and formed withvarious parts thereof reliably fit in each segment and hence, with theentire shape thereof kept properly.

INDUSTRIAL APPLICABILITY

The radial tire with a circumferentially spirally wound belt layeraccording to the present invention is suitable for use as tires forwheels of motor vehicles.

1. A radial tire having a body ply extending from a tread section tosidewall sections and turned up over bead cores of bead sections to haveturned-up portions at both sides thereof; a belt layer wound on an outercircumference of the body ply in a tire circumferential direction; atread circumferentially wound on an outer circumference of the beltlayer; and bead fillers extending from the bead cores radially outwardbetween the sidewall sections and the turned-up portions of the bodyply, wherein: the belt layer comprises at least one spirally wound beltlayer in which one or plural belt cords coated with rubber are arrangedto be spirally wound substantially in parallel with the tirecircumferential direction; and the belt cord in the spirally wound beltlayer has a property that an increase rate of tensile load to stretchrate is small in a range of a predetermined stretch rate or less but islarge in another range exceeding the predetermined stretch rate, andwherein the belt cord in the spirally wound belt layer has a propertythat the stretch rate is equal to or greater than 0.5% at tensile loadof 20 N, and wherein the tensile load is equal to or greater than 60 Nat stretch rate of 3% of the tensile load or is equal to or greater than30 N at the stretch rate of 1.5%, wherein: the body ply is arranged tobe at least one layer; body ply cords of the body ply are inclined at anangle in the range of 85 to 90 degrees relative to the circumferentialdirection; and the belt layer arranged on the outer circumference of thebody ply comprises two oblique belt layers in which belt cords made ofsteel are arranged to be inclined in the same direction at the angle inthe range of 30 to 60 degrees or in the range of 40 to 50 degreesrelative to the circumferential direction and at least one spirallywound belt layer in which a belt cord made of a hybrid cord is coatedwith rubber and is arranged to be spirally wound on outer circumferencesof the oblique belt layers substantially in parallel with the tirecircumferential direction; and of the two oblique belt layers, oneoblique belt layer is in the range of 40 to 70% of the other obliquebelt layer in width and is circumferentially arranged at a centerportion in a direction of the radial tire width.
 2. The radial tire withthe circumferentially wound belt layer as set forth in claim 1, whereinthe belt cord in the spirally wound belt layer includes a hybrid cordhaving a nylon fiber bundle as a core and also includes an aramid fiberbundle twisted therearound.
 3. The radial tire with thecircumferentially wound belt layer as set forth in claim 1, wherein: anorganic fiber cord made of a hybrid cord is coated with rubber andarranged to be spirally wound between the oblique belt layers and theouter circumference of the body ply substantially in parallel with thecircumferential direction thereby to form a ply under the oblique beltlayers; and the organic cord in the ply under the oblique belt layers isarranged to be densified at shoulder portions on side edges in thedirection of the radial tire width and to be loose at a center portion.4. The radial tire with the circumferentially wound belt layer as setforth in claim 1, wherein a ply under the oblique belt layers iscomposed of: a waved steel cord coated with rubber and arranged to bespirally wound and to be densified between the oblique belt layers andthe outer circumference of the body ply at shoulder portions on sideedges in the direction of the radial tire width substantially inparallel with the circumferential direction; and an organic fiber cordmade of a hybrid cord coated with rubber and arranged to be spirallywound and to be loose between the oblique belt layers and the outercircumference of the body ply at the center portion substantially inparallel with the circumferential direction.
 5. The radial tire with thecircumferentially wound belt layer as set forth in claim 1, wherein: acord which is small in the increase rate of tensile load to stretch ratein the range of a predetermined stretch rate or less but is large in theincrease rate of the tensile load in another range exceeding thepredetermined stretch rate is coated with rubber and is arranged to bespirally wound between the oblique belt layers and the outercircumference of the body ply substantially in parallel with thecircumferential direction thereby to form a ply under the oblique beltlayers; and the cord in the ply under the oblique belt layers isarranged to be densified at shoulder portions on side edges in thedirection of the radial tire width and at a center portion, but to beloose at a portion between each shoulder portion and the center portion.6. The radial tire with the circumferentially wound belt layer as setforth in claim 5, wherein: a cord which is small in the increase rate oftensile load to stretch rate in the range of a predetermined stretchrate or less but is large in the increase rate of the tensile load inanother range exceeding the predetermined stretch rate is coated withrubber and is arranged to be spirally wound between the spirally woundbelt layer and the oblique belt layers substantially in parallel withthe circumferential direction thereby to form a ply under the spirallywound belt layer; and the cord in the ply under the spirally wound beltlayer is arranged to be densified at shoulder portions on side edges inthe direction of the radial tire width and at a center portion, but tobe loose at the portion between each shoulder portion and the centerportion.
 7. The radial tire with the circumferentially wound belt layeras set forth in claim 1, wherein both side ends of the spirally woundbelt layer cover both side ends of the oblique belt layers.